Large-scale production of salmon and trout fry in hatcheries has been carried out in essentially the same manner for over a century, and the basic procedures and components have changed little. In recent years, it has become increasingly apparent that although the survival of eggs to the fry stage is much better when the eggs are incubated in hatcheries than in nature, the quality of the hatchery fry is often poor. The low quality of the fry is not as significant in species protected and reared in the hatchery for a few weeks or months (such as chinook and coho salmon), but it may be a critical factor in species released to the wild as fry (such as pink and chum salmon). In recent years, efforts have been made by many experimenters to develop incubation systems that would produce higher quality fry at less cost and at remote sites or sites inaccessible during winter.
Gravel is the natural substrate for incubating salmon eggs and alevins. Although eggs can be incubated successfully in trays or other types of containers with smooth substrates, alevins tend to be more active on smooth substrate than on gravel. This increased activity leads to premature swimming and contributes to poor conversion of yolk to body tissue, resulting in undersized and frequently abnormal fry. New types of hatchery systems, called gravel incubators, can avoid these problems. Gravel incubators do work, but because of the high cost of labor, materials, and transportation in remote areas, e.g. Alaska, a system is needed that requires a minimum of lightweight material and can be transported by air at relatively low expense.